It is already known in the art to control processing operations on a travelling material strip by means of control marks printed on the strip which can be detected by photocells or other optical devices. These patterns which are disposed on the material strip are usually printed in a color tone which contrasts with the surroundings, thus enabling the detection means to detect the exact position of the material strip. These patterns can also be made with magnetically sensible material or with the help of patterns sensible by mechanical means, for example holes, crease lines, slots etc.
By the detection of the exact position of the material strip it is possible to apply multicolour printing on the material strip and to adapt the position of the pictures. These pictures contain several colours which are printed on the material strip by the usage of several processing machines, each of them printing a certain colour on top of the picture in process which has been prepared by preceeding processing machines which have printed other colours on the material strip. Therefore, a multicolour picture can be created and printed on the material strip wherein the different colours overlay each other at the exact position.
Another use of the checking device or checking method, respectively, is to detect the exact position of the material strip in order to apply a certain printing pattern on top of the previously preprinted material strip at a certain point or in order to arrange a printed pattern and a crease line pattern which facilitates the folding in register with each other.
Further fields of application exist, for example, where a packaging material strip is to be advanced in a packaging machine or the like over exactly the length of pattern so as to obtain, on the one hand, the printed pattern in the same position on all packages, and to ensure, on the other hand, that the said crease line pattern coincides with the forming device of the packaging machine so that the folding of the material takes place along the crease lines which are predetermined in their position. In order to manufacture the material strip, like stamping holes, applying opening apertures or additional crease lines, it is necessary to know the exact position of the material strip as well.